In many fluid handling systems it is necessary to provide exceptionally large enclosed conduits for conveying flue gases, smoke, air and other fluids, a typical example of such a conduit being a 20 foot-by-20 foot retangularly shaped conduit used to convey flue gases in conjunction with a power plant precipitator.
Because of the size and service requirements of these conduits, they must be structurally sound, particularly at the corners where full penetration moment connections are generally required, and it is therefore highly desirable from both economic and structural standpoints to weld the corners of such conduits in a fabricating shop rather than in the field. However, when the size of a finished conduit exceeds the existing size limitations imposed on items shipped by trucks or rail, it has not been feasible to construct a finished conduit in a fabricating shop and then ship such conduit intact, in longitudinal sections, by conventional truck and rail transportation. For example, items shipped by rail cannot have a width exceeding 9 feet or a height of 15 feet, and items shipped by truck cannot have a width exceeding 8 feet or a height of 12 feet, and it will therefore be apparent that the aforementioned typical large conduit having having cross-sectional dimensions of 20 feet-by-20 feet cannot be shipped, in finished form, by either rail or highway transportation.
In addition to being used as a fluid conveyor, as discussed above, large structural units of this general type may be used in a variety of other applications, such as the construction of buildings, bridges and other such structures, and they may also be used in the formation of tunnels, mine shafts and the like. Because of their size, structural units of this type cannot be fabricated as a completed unit and then shipped because they would exceed existing transportation size limitations, and they would therefore encounter the same transportation difficulties set forth above.
Accordingly, it is now common practice to partially fabricate a large rectangular structural unit in a fabricating shop by forming a plurality of flat panel sections having dimensions which permit them to be shipped to the job site by conventional rail or highway transportation, and then welding these panels together at the site when they arrive. Because, as mentioned above, the corners of the finished conduit must usually have full penetration moment connections to withstand service loads, the welding of the four corners of the finished duct involves relatively difficult field fitting and welding techniques that must be carried out by experienced welding personnel who must be available at the job site. In addition to the inherent difficulties of field welding as compared with welding at a fabricating shop designed for this purpose, this field welding also requires the presence, at the job site, of experienced welding personnel, all of which adds significantly to the cost of producing a finished large structural unit.
By substantial contrast, the present invention provides a means of specially forming a large structural unit so that it consists of sections that can be readily transported to a job site, and that can then be structurally joined at the job site by relatively simple connections and welding techniques that do not require a significant number of experienced welding personnel.